A quick fix is pretty much impossible... perhaps finding another person with a D620... or talk to Dell.
Unless you are the registered owner, Dell will not help, and it takes about 21-30 days to get registered.
The Dell Latitude D620 is a difficult password fix.
Suggest you read through all the Dell password messages on this TechSpot forum, and there are at least 800 messages... in there somewhere, there may ba a clue, and there may be a person who can repair it... but you may have to ship it elsewhere to get it done. The answer, if there is one, is somewhere in our log of Dell password messages.
You may get lucky, and have the one or two experts who can fix it read your message and contact you.
A fix from a professional service will cost as much as $450... Best place I know of is in Ontario, Canada.